Abstract: Cell, whose interior (3) is filled with liquid, whose radioactivity is measured by a scintillator or other detector install thereon. The invention consists of equipping the cell with rinsing means in order to wash the cell side wall (2) and the sealing or blanking plate (13) with respect to impurities deposited thereon and interfering with the measurement as a result of their natural radioactivity. Blowing means are provided above the free surface of the radioactive liquid in order to oppose its vaporization, which would produce other deposits.

Abstract: Apparatus for measuring the .alpha. activity of a solution and used for determining the plutonium concentration of a solution sampled from an irradiated nuclear fuel reprocessing installation and which comprises a rotary drum (20), which samples a film of solution in a vessel (20), in order to place it in front of a measuring probe (22). Between the probe (22) and the drum (20) is positioned a sealing disk (40) having three positions, which has an effective measuring region (48), a measuring chain quality checking region and a blank measuring region. In order to avoid a solution deposit by condensation on the disk (40) or on the end of the probe (22), a protective gas injection system (49) having passages traversing the vessel cover (16), scavenges the upper face of the sealing disk at three locations corresponding to the three active regions of the latter. A substantially uniform distribution of the gas flow between said three locations is ensured. The protective gas is then sucked in by a tube (122).